Increasing the polysulfide content of an alkaline pulp impregnation liquor

ABSTRACT

A METHOD FOR INCREASING THE POLYSULFIDE CONTENT OF AN ALKALINE IMPREGNATION LIQUOR. AN IMPREGNATION ZONE IS DIVIDED INTO A LOWER TEMPERATURE ZONE AND A HIGHER TEMPERATURE ZONE. THE LOWER TEMPERATURE ZONE IS MAINTAINED AT A TEMPERATURE BELOW THAT AT WHICH DEGRADATION OF POLYSULFIDE OCCURS. AFTER AN INITIAL CONTACT OF CHIPS AND IMPREGNATION LIQUOR, THE LIQUOR IS WITHDRAWN FROM THE HIGHER TEMPERATRUE ZONE AND FROM CONTACT WITH THE CHIPS, OXIDIZED TO FORM A LIQUOR CONTAINING POLYSULFIDES, AND THEN RETURNED TO THE LOWER TEMPERATURE ZONE OF THE IMPREGNATION ZONE FOR FURTHER IMPREGNATION OF THE CHIPS.

30, 1971 w. H. COOK 3,573,157

-' INCREASING THE POLYSULFIDE CONTENT OF AN ALKALINE PULP IMPREGNATION LIQUOR Filed May 8, 1967 2 Sheets-Sheet 1 F 2 INVENTOR William Hugh COOK PATENT/4 T W. H. COOK ASING THE POLYS KALINE PULP IMPR Much 30, 1971 UL E CONTENT OF AN E TION LIQUOR 2 Sheets-Sheet 2 Filed May 8. 1967 FIG. 4

United States Patent 3,573,157 INCREASING THE POLYSULFIDE CUNTENT OF AN ALKALINE PULP IMPREGNATION LIQUOR William Hugh Cook, Beacoustield, Quebec, Canada, assignor to Domtar Limited, Montreal, Quebec, Canada Filed May 8, 1967, Ser. No. 636,874 Int. Cl. D21c 3/26, 11/14 U.S. Cl. 162-19 2 Claims ABSTRACT OF THE DISCLOSURE The present invention relates to a pulping process using an alkaline liquor having a high content of polysulphide. The invention relates, more particularly, to an alkaline pulping process wherein polysulphide is formed in the system by oxidation of the liquor at an intermediate stage of the process.

It is well known that the yield of pulp in kraft cooking (for a given degree of delignification) is increased when alkali polysulphides are present in the cooking liquor. The polysulphides have been generally prepared by reacting elemental sulphur with a solution of alkali metal monosulphide; in practice, liquors containing polysulphides have been made, e.g. by adding elemental sulphur to the green liquor (containing predominantly alkali carbonate and sulphide) or to the white liquor (containing alkali metal hydroxide and sulphide) in the kraft process. Such addition of sulphur, however, is known to disturb the sulphuralkali balance in conventional processes, unless the amount of sulphur added is relatively small. This happens because, after digestion of the wood and combustion of the black liquor, the sulphur added appears in the form, not of polysulphide but of sulphide, and this leads to an accumulation of sulphur in the system greater than the system can cope with, and in consequence to excessive odour problems etc.

It has been proposed to form polysulphides in a sulphate liquor by a limited oxidation of mixtures of black liquor and white liquor, thereby to form polysulphides from the monosulphide present in such mixtures. (Essentially no polysulphide is formed when only white liquor is oxidized.) Mixtures of black liquor and white liquor in proportions of about 1:1 have been proposed. However, in systems with high sulphur loadings, such as are inherent in pulping with polysulphide, the recycling of black liquor results in a further aggravation of the odour problem. The present invention provides a method of pulping with liquors containing polysulphide without the necessity of recycling black liquor, and is particularly suitable for rapid cycle continuous processes e.g. the vapour-phase process.

The present invention is particularly adapted for use in those pulping processes using a kraft liquor or liquors of higher sulphidity, which include a discernible impregnation step, i.e. where the chips are maintained for a time, which may be of varying length, in contact with the cooking liquor, usually under superatmospheric pressure, but at a temperature not high enough for any substantial cooking of the chips to occur within such length of time. In some processes the impregnation step is carried out as a separate and clearly defined step, forming, of course, part of the whole sequence of steps in the process. In one known process, for example, the chips are impregnated with the cooking liquor in an impregnation stage until they have retained a desired amount of chemical and then forwarded to a cooking stage which may conveniently take place in a separate vessel. However, it is possible to carry out impregnation and cooking of the chips in the same vessel, e.g. by providing for relatively distinct temperature Zones in a column of liquor through which the chips are made to travel, or by carrying out the impregnation and cooking in a time sequence, or the like.

The present invention provides for the oxidation of liquor Withdrawn from an impregnation zone to form polysulphides and the return of such oxidized liquor to the system for impregnation and cooking of chips.

It has been found that a liquor which has been used for the impregnation of chips will, when subjected to oxidation, form polysulphides in substantial amounts. This was a surprising discovery in view of the known fact that white liquor, on oxidation, does not produce essentially any polysulphide, and that to obtain any significant yield of polysulphide it was hitherto necessary to use mixtures of black and white liquors. The impregnation liquor to be oxidized according to this invention will have been in contact with the chips in conditions of temperature and time such as are usually employed for impregnation, viz sufficient for an initial chemical interaction between the liquor and the wood to take place without any substantial deligniflcation of the wood. It has been further found that the amounts of polysulphide formed by oxidation of impregnation liquors according to this invention are generally higher than those formed in the mixtures of black and white liquors of the prior art. It is believed that polysulphides are obtained more readily in an impregnation liquor according to this invention because such liquors have generally a high concentration of hydrosulphide ions relative to the caustic, and the caustic content is relatively low. Where the impregnation is carried out continuously, e.g. where the chips are continuously passed through a pool of liquor wherein they are impregnated with the liquor, there will be a tendency for the liquor in the pool to become enriched in hydrosulphide ions, due primarily to the preferential sorption of sodium hydroxide, compared with sodium hydrosulphide, by the chips and also to the consumption of sodium hydroxide for the neutralizing of acids dissolved out from the wood at the prevailing temperatures. When a steady state has been reached in the impregnation zone, the concentration of hydrosulphide in the liquor surrounding the chips in the impregnation zone will generally be considerably higher than the corresponding concentration in the white liquor fed to the system, and much higher still than the concentration of hydrosulphide in. the black liquor. Such comparatively increased concentrations of hydrosulphide in the liquor will be found in the impregnation zone irrespective of whether the white liquor fed to the system is a conventional kraft liquor of about 25-30% sulphidity or a liquor of higher sulphidity up to sulphidity. It will be appreciated that if for any reason it is desired to add elemental sulphur to the system (e.g. to make up sulphur losses) addition of sulphur to the recycled portion of the impregnation liquor will result in additional generation of polysulphide.

Further features, objects and advantages of the present invention will be evident from the following detailed 3 description of specific systems taken in conjunction with the accompanying drawings in which:

FIG. 1 is a schematic representation of one system for carrying out the present invention utilizing one type of apparatus.

FIG. 2 illustrates an arrangement similar to that in FIG. 1.

FIG. 3 schematically illustrates another system according to the present invention and utilizing a second form of apparatus.

FIG. 4 schematically illustrates yet another form of the invention.

It will be understood that this illustration, or the examples recorded below, are not to be interpreted as a limitation of the invention to the specific embodiments disclosed.

In the embodiment of FIGS. 1 and 2 material to be impregnated enters the impregnator 12 through inlet 7 and is carried through the pool of impregnation liquor by flight conveyor 16 downwardly along the upper surface of the midfeather 14 and upwardly along the lower surface of same to the outlet 8 leading to the digester vessel 9.

In the embodiment in FIG. 1, impregnation liquor is withdrawn from the pool 10 in the impregnation vessel 12. This vessel 12 has a midfeather 14 and flight conveyor schematically illustrated at 16. The liquor withdrawn from the pool 10 is conducted via lines 18 and 20 to a heat exchanger 22 which cools the liquor, and via line 24 to the oxidation tower 2. Oxidized liquor from the tower 2 travels via line 26, heat exchanger 22, and lines 28 and 30 back to the pool 10. As shown, the tower 2 is provided with a conventional recirculation line 2a.

In the FIG. 1 arrangement, part of the withdrawn liquor flows via line 32, heat exchanger 34 and line 36 to flash tank 38 and from the flash tank 38 via line 40 to line 30 and hence to the pool of liquor 10. The heat exchanger 34 and flash tank 38 operate to maintain a water balance in the system in the manner described in Canadian Pat. 721,960. If, for some reason, it is not desired to maintain a water balance the above described flash system may be omitted and the circulated liquor heated in any suitable manner.

Fresh make-up liquor is added in any suitable manner, for example, through line 5 connected to line 40. Generally, this make-up liquor will be white liquor but it may contain admixtures including, if so desired, for any reason, admixtures of white liquor with some black liquor.

Material to be impregnated enters the impregnator 12 through inlet 7 and is carried through the pool 10 via flight conveyor 16 downwardly along the upper surface of the midfeather 14 and upwardly along the lower surface of same to the outlet 8 leading to the digester vessel 9.

In the embodiment of FIG. 2 the impregnation zone is substantially the same as that described above but the system for flashing to maintain a water balance has, in the FIG. 2 embodiment, been combined with cooling of the liquor to be oxidized. The system of FIG. 2 makes available only low pressure steam and would generally only be suitable in special circumstances.

In the FIG. 2 arrangement, liquor withdrawn from the impregnation zone 10 via line 18 is controlled in accordance with the level in the pool by means of sensor 44 controlling pump 42. This withdrawn liquor is flashed in sump tank 46 and the steam so formed is removed via line 48. Liquor is fed from the tank 46 to the oxidation tower 2' via line 50 where it is oxidized to form polysulphides and returned to the tank 46 via line 52. The level of the tank 46 is held substantially constant by sensing means 54 controlling pump 56 which pumps liquor through heat exchanger 58 and back to the impregnation zone via line 60.

Another type of apparatus wherein impregnation and cooking are carried out in the same vessel has been illustrated in FIGS. 3 and 4, however, it is evident that the modes of operation described with respect to one apparatus could, with suitable modification, be carried out in another apparatus. In FIGS. 3 and 4 a vertical tubetype digester has been illustrated, i.e. a vessel divided by suitable flow control means into discrete zones for impregnation and cooking etc.

The two zone impregnation system schematically shown in FIG. 3 consists of an upper of first zone 72 and a second zone 74 contained within the continuous digestion vessel 70. The first zone is at a lower temperature than the second zone to facilitate impregnation with the polysulphide containing liquor. As illustrated, the impregnation liquor and chips travel co-currently through the first zone 72 and counter-currently through the second zone 74 with the liquor travelling upward through the second zone.

As shown in FIG. 3, impregnation liquor is withdrawn from the vessel 70 via line 76 and a portion of this liquor passes via line 78 and heat exchanger 80 (where it is cooled) to oxidizer 82, having a recirculation line 82a. Oxidized liquor leaves the oxidizer 82 and travel-s via line 84 through heat exchanger 80', a supplemental heater (not shown), if necessary, and into the top of zone 72 via line 86. The liquor in line 86 has a high polysulphide concentration relative to the other liquor in the system.

After this high polysulphide content liquor has passed through the zone 72, it is removed via line "88 which connects with line 90 carrying liquor from the impregnation zone 74 and flows into the heat exchanger 92. Alternatively all or a portion of the liquor in line 88 may be directed to line 78. Heated liquor from the exchanger 92 is returned to the bottom of the zone 74 via line 94 and flows upwardly through the zone 74 to be removed at 76. Flow through the various lines may be adjusted according to the amount of polysulphide to be produced and the specific process to be carried out. For example, the amount of liquor withdrawn via line 88 need not be equal to that entering the zone '72 through line 86 so that some of the liquor from zone 72 may carry over to zone 74 and be removed via line 76. If desired, a flash system may be included and controlled to maintain a water balance in the system. In the illustration embodiment, make-up liquor is added via line 96.

In the arrangement of FIG. 4 the material to be cooked flows downwardly through the tubular vessel through a low temperature impregnation zone 102 and a high temperature impregnation zone 104, arranged in series, and then into a digestion zone. Liquor is withdrawn from the vessel 100 at the bottom of zone 104 via line 106, is heated in heat exchanger 108 and returned to the zone 104 via line 110. A portion of the withdrawn liquor is directed via line 112 to join liquor withdrawn from low temperature zone 102 via line 114 and to flow via line 116 through heat exchanger 118 and line 120 into oxidation tower 122. The tower 122 is provided with a conventional recirculation line 122a. Oxidized liquor from the tower 122 passes via line 124 through heat exchanger 118 and a supplemental heater (not shown), if required, via line 126 to the top of zone 102. Make-up liquor may be added to the withdrawn liquor at any suitable location such as by line 128, also a flash system for maintaining the water balance may be included if desired.

The high polysulphide content liquor enters the top of zone 102 via line 126 and flows downwardly through the zone 102 with a portion of this liquor being withdrawn via line 114. As the chips and liquor proceed downwardly into the high temperature zone 104 they are heated by liquor entering the top of the zone 104 via line 110. The impregnated chips leave the zone 104 and flow into a generally higher temperature digestion zone.

Following are some examples, illustrating but not limiting, the practice of the present invention.

5 EXAMPLE I Samples of different liquors were oxidized, the composition of the liquor before and after oxidation being as follows:

COMPOSITION OF THE LIQUOR Before oxidation After oxidation Poly- Poly NaHS sulphide NaI-IS sulphide g.p.l. as g.p.l. g.p.l. as Eff g.p.l.

NazO alkali as S N 2.20 alkali as S 1. Solution of NazS 54 54 3 1. 1

2. Impregnation liquor 67. 3 26. 0 4. 0 39. 5 36. 8 21. 5 3. 50% black liquor and 50% feed liquor 38. 3 28. 8 9. 3 36. 2 7. 2

Liquor No. 1 is merely a solution of sodium sulphide. Liquor No. 2 was withdrawn from the impregnation tube of a digester of the type described in Canadian Pat. 721,- 960. Liquor No. 3 was withdrawn from the cooking tube of the same digester system and consists of exuded and black liquor and condensed steam substantially as described in Canadian Pat. 727,001. The liquors were subjected to oxidation in an apparatus known as a Turbulent Contact Absorber (as described in British Pat. 1,020,- 483) at a temperature of about 70 C.

Liquor No. 1 corresponds to the 'white liquor in a 100% sulphidity cook and it is evident that the white liquor does not lend itself to formation of polysulphide by oxidation. A comparison of the impregnation liquor No. 2 with the mixture of black liquor and white liquor No. 3 from the same cook shows the advantage of oxidiz ing impregnation liquors as a source of polysulphide for use in the process.

EXAMPLE II In an arrangement substantially as indicated in FIG. 2 and after 30 hours of operation the liquor withdrawn from the impregnation vessel in line 18 contained 5.6 grams per liter polysulphide sulphur while the liquor returned to the impregnation zone from the sump 46 contained 17.5 grams per liter polysulphide sulphur for a net pick-up of almost 12 grams per liter of polysulphide sulphur.

I claim:

1. In a continuous method of pulping cellulosic raw material wherein said material is first impregnated with a digestion temperature in a cooking zone, the improvement comprising maintaining in said impregnation zone a lower temperature zone and a higher temperature zone, maintaining in said lower temperature zone a temperature below that at which substantial degradation of polysulphide occurs and maintaining in said higher temperature zone a temperature higher than in said lower temperature zone, withdrawing impregnation liquor after contact with said material from said impregnation zone, oxidizing at least a portion of said Withdrawn impregnation liquor to form an oxidized liquor containing polysulphide and returning said oxidized liquor into said lower temperature zone of said impregnation zone.

2. A method as defined in claim 1 wherein said impregnation liquor is withdrawn from said higher temperature zone.

References Cited UNITED STATES PATENTS 3,193,444 7/1965 Benjamin 16282X 3,216,887 11/1965 Landmark 16282X 3,303,088 2/1967 Gessner 16241X 3,448,002 6/1969 Bryce et al. 16243X 3,470,061 9/1969 Barker 162-32 S. LEON BASHORE, Primary Examiner A. DANDREA, 111., Assistant Examiner US. Cl. X.R. 162--29, 35, 42, 82 

